This invention relates to compensation networks for use with feedback control systems. More particularly, the invention relates to a method and apparatus for alteration or adjustment of a control system, by use of a compensator, to obtain optimum response in a closed feedback loop and for providing a reading of the nature of the compensator employed.
Feedback control systems, such as closed loop servo systems, are commonly used to control such things as the position and velocity of a motor. Such systems are typically defined by their system transfer function. In the case of a motor used in a device, such as a robot or the like, the internal operating parameters of the motor are elements of the system transfer function.
A process, represented by the transfer function G(s), in LaPlace transform notation, whatever its nature, is subject to a changing environment, aging, ignorance of the exact values of the process parameters, and other natural factors which affect a control process. It is not always possible to simply adjust a system parameter and thus obtain the desired performance. Instead, it is necessary to reconsider the structure of the system in order to obtain a suitable one.
In redesigning a control system, in order to alter the system response, an additional component is inserted within the structure of the feedback system. It is this additional component or device that equalizes or compensates for the performance deficiency. The compensating device may be an electric, mechanical, hydraulic, pneumatic, or other type of device or network, and is often called a compensator. Commonly, an electric circuit serves as a compensator in many control systems. The transfer function of the compensator is designated as G.sub.c (s)=E.sub.out (s)/E.sub.in (s) and the compensator may be placed in a suitable location within the structure of the system. Several types of compensator are shown in FIGS 1a-1d for a single loop feedback control system. The compensator placed in the feed forward path is called a cascade or series compensator. Similarly, the other compensation schemes are called feedback, output or load and input compensation, as shown in FIGS. 1(b ), 1(c ) and 1(d ), respectively. The selection of the compensation scheme depends upon a consideration of the specifications, the power levels at various signal nodes in the system, and the networks available for use.
In the past, the designers of systems were limited in knowledge of loop component transfer functions and were forced to rely on approximate transfer functions until a feedback control system was constructed. After the control system was constructed a Bode plot (frequency response transfer function) would be obtained on the actual hardware of the system, in order to determine the small and large amplitude stability margins. At this juncture, a compensation network could be designed in an attempt to obtain desired performance. Typically, changing the performance of a system involved pulling out circuit boards and rearranging components, or replacing old circuit boards with new ones. Unfortunately, this process involves an unreasonable amount of guess work and a great deal of engineering time--as the precise changes needed are not at all apparent. Often, the theoretical compensator differs from the compensator actually needed to obtain desired performance. Further, because similar systems may require different compensation, the engineering time needed to obtain desired performance may be prohibitive.